Key to the phony Windows Update process was that the hackers had located and exploited a flaw in the company's Terminal Services licensing certificate authority (CA) that allowed them to generate code-validating certificates "signed" by Microsoft.

Armed with those fake certificates, the attackers could fool a Windows PC into accepting a file as an update from Microsoft when in reality it was nothing of the kind.

"Hijacking Windows Update is not trivial because updates must be signed by Microsoft," noted Symantec on Monday in one of a series of blog posts its researchers have written about Flame.

One of the certificates was valid between February 2010 and February 2012, and used to sign the malicious file in late December 2010, adding more information to experts building a timeline of Flame's development and attacks.

Other security experts were even more impressed with what Flame managed. Earlier Monday, Mikko Hypponen, F-Secure's chief research officer and the first to announce that Flame was abusing Windows Update, called the feat "the Holy Grail of malware writers" and "the nightmare scenario" for antivirus researchers.

Instead, a Flame-infected Windows PC can, in some situations, make other machines on a network believe it's Windows Update.

A PC compromised by Flame can sniff a networks' NetBIOS information, which identifies each computer, then use that to intercept Windows Updates requests by Internet Explorer (IE). Flame claims to be the WPAD (Web Proxy Auto-Discovery Protocol) server -- a system that provides proxy settings to copies of IE on the network -- and sends a malicious WPAD configuration file to the requesting PC.

As Symantec noted, WPAD hijacking is not new and is, in fact, part of many hacker toolkits.

The rogue WPAD configuration file modifies the victimized machine's proxy settings so that all Web traffic is routed through the Flame-infected system. On that PC, Flame's Web server, dubbed "Munch" kicks in, detects when the requested URL matches Windows Update's and in return sends a downloader disguised as a legitimate update from Microsoft.

To complete the ruse, the downloader was one of several compressed files -- crunched into the "cabinet," or ".cab" file format -- bundled into the single Windows Update.

Once the downloader was installed it retrieved a copy of Flame from the already-infected PC and uses it to compromise the computer.

This complex spreading technique only added to researchers' grudging respect for the threat.

"As we continue our investigation ... more and more details appear [that show] this is one of the most interesting and complex malicious programs we have ever seen," said Alexander Gostev, who leads Kaspersky's research and analysis team, in a Monday blog entry.

Microsoft has revoked three certificates generated by the attackers, making further spoofing of Windows Update files impossible on patched PCs unless there are more rogue certificates in the wild. The company has also blocked others from cranking out new code-signing certificates.